The moisture - bleach - will seep into the grout slowly, all the better to perform the bleaching.  

(be gentle :shock: )

Stick with your job and hire all that out.Really --and I know this isn't where you were going with this -- you have to figure out WHY you're doing this, as @Steve Vaughan gently keeps trying to point out in his posts.

Your FIL probably hadn't jumped into overalls and crawled underneath the house - any contractor that does and who has your interests in mind would be able to point out first signs of termites, mold, moisture and things like obvious cracks in foundation that old houses are famous for.

It probably has a 1/4" screw you need to remove, pull it out and gently clean with sandpaper.

I have also applied it to the shower stall ceilings to avoid moisture issues & paint peeling.

I am NOT aware of anybody who has died from exposure to this mold.WHY MOLDS KILLMolds grow in response to moisture.  

If you’re finishing a basement, you’ll have to pay major attention to any issues with moisture.

The house was in way worse shape (including lots of moisture damage from an indoor marijuana grow), however our rehab was at a much lower finish level.  

I let them down as gently as I can.. these are folks that just don't really know how RE lending works ..